The Volumetric Variability of Subtropical Mode Water in the South Atlantic

Time series of global monthly objective analyzed fields of temperature and salinity between 2002 and 2014, provided by the In Situ Analysis System (ISAS), were analyzed to detect and quantify the volume of the subtropical mode water (STMW) in the South Atlantic ocean. The STMW is identified as homogeneous layers in the temperature range of 14,1 and 18.5°C, salinity between 34,7 and 36,5, and low potential vorticity. The selected profiles were classified into two categories: the ones at the surface and those entirely in the subsurface. The first ones are related to profiles that depict the formation of STMW, and thus outcrops at surface, the ocean-atmosphere interface. The formation process is restricted to winter and early spring months (July to October) in an area between 32°S and 40°S and west of 20°W. In contrast, the profiles of the second category are found below the surface but above the main thermocline and throughout the year. By estimating the volume by month of each category, it became feasible to highlight the relationship between the amount of STMW formed and the amount immersed, and the interannual variability. During the formation months the layer thickness is on average 135,6 ±2,4 m deep and can reach up to 300 m. The average volume of STMW at surface is 0,53 (±0,16)×10¹⁴m³ with a maximum of 2,56×10¹⁴m³ in 2010. The submerged profiles led to the estimation of an average volume of 1,20 (±0,04)×10¹⁴m³ during the study period, with maxima near the end of formation months and minima during early winter just preceding the formation period. This reduction of the subsurface STMW volume throughout the year is due to internal dissipation processes or even lateral advection. After its formation, the water sinks and moves northwestward towards the center of the subtropical gyre, covering a broader area between 40°S e 25°S, however with the mean thickness reduced to 150 m. The phase lag between the volume time series determined for each category is about 3,3 months.